I'm curious what you see as the reason that putting everything behind multiple layers of nat can't work? It seems to me like it has worked pretty well so far, and we're nowhere close to running out of (ip, port, ip, port) tuples.
1 layer of nat on each side hasn't bad, the 2 layers of nat on each side carriers have been moving to has been a godawful mess of complexity for any conversation where at least one side isnt a public IP (e.g. p2p chat/calls).
